Telephone signaling system



1951' 5 sheet-snet 1 Aug 9, 1932- L., G. ABRAHAM ET AL TELEPHONE SIGNALING SYSTEM Filed Nov. 21

i@ iwf/ales l INI L. G. ABRAHAM ET AL. 1,870,395

TELEPHONE SIGNALING SYSTEM Filed Nov. 2l, 1951 5 Sheets-Sheet 2 Aug. 9, 1932.

Aug. 9, 1932. L. G. ABRAHAM ET Al.

TELEPHONE SIGNALING SYSTEM Filed NOV. 21, 1931 3 Sheets-Sheet 3 INVENTORS 6, @14am/n,

Patented Aug. 9, i 1932 UNITED STATES PATENT carica LEONARDv G. ABRAHAM, OF MADISN, AND ELLOT STAPLES, F HOHOKUS, NEW

JERSEY, ASSIGN ORS TO AMERICAN TELEPEOIT E AND TELEGRAPH COMPANY, A COR- IPORATON OF NEW YORK TELEPHONE SIGNALING sYsTEM Application led November 21, 1931.

This invention relates to telephone systems and more particularly to improvements in signaling arrangements adapted to be associated with the lines of such systems.

The signaling arrangements of the invention are particularly adapted for toll circuits in a telephone system. When utilized with such circuits they permit any given operator, for example, the operator originating the call, to .completely supervise the call. Automatic ringing, switch hook operation, and the completion of the call would all be under the supervision of this one operator. The arrangements of the invention differ from other signaling systems in that they permit the signals to be passed over a circuit in two directions simultaneously when it is not in use. The signaling circuits of the invention may furthermore be used in built-up connections f without repeating the signals at thel junction points of the lines. Another advantageof the signaling arrangements of the invention consists in the fact that they do not require signals of definite length or lock-up devices.

Other objects and features of the invention will appear more fully from the detailed description thereof hereinafter given.

Themethod of signaling of the arrangements of the invention is accomplished by 'n providing two sending and two receiving ringer-s at each end of a long distance line. These sending and receiving ringers would utilize signaling currents of different frequencies. For example, one set of ringers might operate with a frequency of 1400 cycles and the other set might operate with a frequency of 1000 cycles. The ringers furthermore are so interconnected that when one frequency is being sent the receiying ringer for that frequency at the same end of the line is disabled before this frequency is applied and only the other frequency may be received at that end of the line. Further details of the system may be more fully understood from the detailed description thereof hereinafter given.

The invention may be more fully understood from the following description together with the accompanying drawings in Figs. 1, 2 and 3 of which the invention is illustrated.

SeraI No. 576,586.

Figures 1, 2 and 3 when taken together with Fig. 2 placed to the right of F ig. 1 and F ig. 3 placed to the right of Fig. 2 show a telephone connection embodying the signaling arrangements of the invention.

1n the drawings is shown a telephone circuit connecting a subscriber tributary to an exchange A with a subscriber tributary to an exchange C. The individual circuits for completing the connection are as follows: A connection would be established at A with the subscribers line thereat (not shown) by means of the cord circuit 13. The cord 13 would be connected to the telephone circuit L interconnecting points A and B. At B the circuit might. -be extended by the cord circuit 20 to the telephone circuit L interconnecting points B and C. At C the call would be extended by the cord 28, the switching trunk 38 and the local exchange cord circuit 39 to the subscribers line 40.

The circuit comprising the line L connecting points B and C will first be described as it is shown in the idle condition, or with no connections thereto. The line L is of the well known four-wire type comprising two one-way line sections 1 and 2 for transmission in opposite directions. rlhese lines would terminate in the hybrid coils 3 and 4" and the balancing networks Nl and Nz and might include the amplifiers A1,A2, A3 and A4. Connected with the line 1 at B would be the sending ringers A and 6. rThe operation of ringer 5 would be cont-rolled by relay 41 and the ground connected to upper contact of jack J 3 and the ringer 6 would be controlled by said ground and by relay 42. Ringer 5 when operated would apply to line 1 current of 1000 cycles and ringer 6 when operated would apply to line 1 current of 1400 cycles. Vihen the, circuit is in idle condition only one of the ringers would be operated at a given time. As shown in the drawings 1000 cycle current is being transmitted over line 1 from B to C. Connected to line 1 at C would be the receiving ringers 9 and The circuits for these ringers would be controlled by relays 43 and 44 and would include the filters Fl and Ff. These filters would make ringer 9 responsive to 1400 cycle to conductor 51 and the relays 45 andl 46 will be operated as shown. These relays control the operation of the sending ringers 11 and 12 connected to line 2 at C. Ringer 11 with the line L.

when operat-ed will send 1400 cycle currenty from C to B and ringer 12 when operated will send 1000 cycle current from. C to B.

These ringers are controlled by relays 45 and 46 and by the ground on the lower contact of jack J4; As shown the ringer 11 will be operated and 1400 cycle current will be sent from C to B. At B there would be provided the receiving ringers 7 and 8. The 1400 cycle current transmitted from C to B would operate the receiving ringer S and the ringer 7 ywould not be operated. This would remove ground from conductor 53 and allow battery to remain connected to conductor 52. Under these conditions the signal lamp 26 would not be operated. Thus it will be seen that when the circuit is in idle condition ringing current of one frequency, such for example as 1000 cycle current. will be transmitted from B to C and ringing currentof the other frequency, such for example as 1400 cycle current will be transmitted from C to B. Under such conditions the signals associated with the line at each end thereof will not be operated.

The telephone circuitL extending between A and B is substantially similar to the circuit L. extending between B and C just described. The circuit L is shownwith certain connections established therewith and is not in the idle condition. For example, the operator at A may connect her cord circuit 13 This will remove the ground from the upper contact of jack J1 and hence remove the ground from the sending ringer 5. This will stop the application of 1000 cycle ringing current to line 1 and the transmission of such current to B and willV release the receiving ringer 10. This will connect battery to conductor 50 and allow ground to remain connected to conductor 49. This will light signal lamp 19 until the B operator plugs in. Before the B operator plugs i'n and removes the ground Jfrom the lower contact of jack J2 the release of the receiving ringer 10 will remove battery from conductor 51 and release relays 45 and 46. The release of relay 46 will remove 1400 cycle current from line 2 and will cause ringer 12 to apply 1000 cycle current to line 2. Thiswill operate the receiving ringer 7y and release the receiving ringer 8. The operation of receiving ringer 7 will remove battery from conductor 52 and allow relay 16 to release. This will allow the lamp 15 in they cord 13 to remain lighted. Then the operator at B in response to the lighting of lamp 19 connects the cord circuit 20 to the line L Y the ground removed from the lower contact of jack J2 and the sending ringer 12 is released as well as the sending ringer 11.

Under these conditions no current is transmitted from B to A and both receiving i ringers 7 and 8 are released as shown. This will cause both relays 16 and 17 to operate and the lamp 15 will be extinguished thus giving the A operator a signal that the B operator has answered.

If the call is for a` subscriber at another point, such as C, the operator at B will connect the cord circuit 20 to the line L extending from B to C. This line has heretofore been described in its idle condition with 1000 cycle current being transmitted from B to C and 1400 cycle current being transmitted from C to B. lVhen the plug of cordy 20v is inserted in jack J3 of line Lthe ground will be removed from the upper contact of jack J3 and ringer 5 will be released. ThisV will stop the transmission of 1000 cycle current between B and Cv and will release the receiving ringer 10. The release of this ringer will apply battery to conductor 50. 95 As ground is already connected to conductor 49 the signal lamp 27 will now be lighted to signalthe operrtor at C. YThe release of ringer 10 will also remove battery from conductor 51 and release relays 45 and 46. 100 The release of relay 46 will stop the transmission of 1400 cycle current from C to B. The release of relay 45 will operate ringer 12 and transmit 1000 cycle current from C to B. This will cause the receiving ringer 8 105 at B to release and put ground on conductor 53 and operate' relay 25. The receiving ringer 7 at B will be operated and remove battery from conductor 52 and allow relay 24to release. However, with relay 25 oper- 110 ated thesignal lamp 21 in tbe cord 20 will be extinguished even if relay 24 is released.

It is pointed out that when the cord 20 was connected` to line L the battery normally connected to conductor 52 would operate 115 relay 24 and extinguish lamp 21. Accordingly. the 1000 cycle current being transmitted from C to B will not signaltbe operator at B.

The 1000 cycle current now being transmitted from C to B will be transmittedover the talking" conductors of the cord 20 and thence over circuit 2 of. line L to point A and will operate the receiving ringer 7 thereat. 1125 The operation of ringer 7 at A will remove battery from conductor 52 and release relay 16. This will light Athe signal lamp 15 in cord 13 andy signal the operator at A' that the call has reached C.

lVhen the operator at C in response to the signal 27 connects the cord circuit 28 with the line L ground will be removed from the lower Contact of jaclr .L and the 1000 cycle sending .ringer 12 will be released. This will stop the transmission of 1000 cycle currentbetween C and A and will release the receiving ringer 7 at A. The release of this ringer will restore battery to conductor 52 and operate relay 16 again; This will extinguish lamp15 in cord 13 anu thus notify the operator at A that the operator at C has connected with the line. lV hen the cord 23 is connected to line T the relays 30 and 31 will be operated but the light 29 will remain on as a circuit therefore will be closed over the contacts of relays 32 and 33.

lf the call is for a subscriber in the region tributary to C, the operator at C may connect her cord circuit 28 to a switching trunk such as 3S. A cord circuit, such as 39, at a local exchange will extend the connection from the switching trunk to the called subscribers line 40. lVhen the cord 28 is connected to the switching trunk 38 the relay 33 will be operated and the lamp 29 at C will be extinguished. Then the connection is established as far as the subscribers line 40 the subscriber may be rung automatically in any ll known manner. When the connection is established as far as the subscribers line and before the subscriber answers the following circuit will be closed,-froin ground connect-ed to the contact of relay 35, conductor 60, in parallel over the sides of trunk 38, conductor 61, conductor 62, conductor 63, to battery in the 1000 cycle sending ringer 12. his will operate the sending ringer 12 and once again send 1000 cycle current from C to A and will operate the 1000 cycle receiving rino'er 7 at A. This will remove battery from conductor 52 and will release relay 16 and light larnp 15 in the cord circuit 13. This will notify the operator at A that the call has been completed to the subscribers line.

when the subscriber removes the receiver 37 and closes the switchhool contacts a circuit will be completed for the relay 35 in the local exchange cord circuit and said relay will operate. rThis will remove the ground from the previously traced circuit for the 1000 cycle sending ringer 12 and cause it to release and stop sending 1000 cycle current from C to A. This will release the receiving ringer 7 at A and cause the relay 16 to be operated again. This will extinguish lamp 15 to notify the operator at A that the subscriber has answered. Obviously, if the subscriber at C flashes his switchhoolr or if busy signals are applied a signal will be received vat A.

When the call is completed and the subscriber at C hangs up the receiver 37, the circuit for relay 35 will be broken and relay 35 operator at A that the subscriber at C has hung up. The operator at A may now withdraw the cordcircuitf13 from the line L.

This will put ground on the upper contact ofY jack J1 and, as the ringer 7 is in operated condition and the relays and 71 released, the 1400 cycle ringer 6 will be operated and will send 1400 cycle current from A to C.

This 1400 cycle current will operate the receiving ringers 9 at B and 9 at C, and will remove ground from conductor 49 at B and 49 at C and release relays 23 and 31. A circuit will then be closed for the lamp 21 at B as follows: from battery 74, armature and contact of relay 23, conductor 75, armature and contact of relay 24, filament of lamp 21, to battery 74. This will light lamp 21 and inform the operator at B that the call is over and to disconnect fromlines L and L. kA circuit will also be closed for the lamp 29 at C as follows: from battery 73, armature and Contact of relay 31, conductor 72, contact and armature of relay 32, filament of lamp 29, to battery. This will light lamp 29 and y notify the operator at C that the call is iinished. The operator at C will then remove her cord 28 from the line L. If the operator at B disconnects from the lines before operator C disconnects, lamp 29 will relight as ringer 6 will be operated to send out 1400 cycles from B and ringer 9 will be reoperated. If operator C disconnects before operator B, lamp 21 will relight as ringer 12 will send 1000 cycles from point C, and ringer 7 will be reoperated. When the operator at A wishes to disconnect the lines from the subscribers station at C when the subscribers receiver is off thehoolr, she removes cord 13 as before. This operates the 1000 cycle sending ringer as relays 70 and 71 are operated due to the receiving ringer 7 being released. This operates ringers 10 at B and 10 at C. Lamp 21 at B will not operate at this time as relay 25 will be operated. The circuit will be closed for the lamp 29 as follows: from battery 73, armature and contact of relay 33, conductor 72, contact and armature of relay 30, filament of lamp 29, to battery. Relay 33 will be released and relay 32 operated, as relay R will be released as relay 35 will be operated. The operator at C will disconnect and this will operateY sending ringer 11 and receiving ringer 8. Lamp 21 will then be lighted by the following connection: from battery 74, to contact and armature of relay 25, to conductor 75, to contact and armature of relay 22, tolamp 21, to battery 74. The

`f B any signal. In addition,

sent and received in the same manner as operator at B will now disconnect. A disconnect signal from A to C can also be transmitted when the subscriber at C is flashing his switchbook or when a busy signalY is being ,sent from C.

end of the Ycircuit for that frequency is disabled and only the other frequency may be received. Also, if 1000 cycles is being received only 1400 cycles may be sent and if either 1400 cycles or nothing is being received, only 1000 cycles may be sent.

It will also be seen from the operator at B` does not receive any signal unless one receiving ringer of each frequency on one or the other of the two circuits is operated, so that signals may be passed in any one direction without giving the operator at if a signal, say of 1000 cycles is passed through the point B operating the 1000 cycle receiving ringer on circuit A--B, and if echoes from circuit B are sufficient to operate the receiving 1000 cycle ringer on that circuit, no signal will be received by the operatorl at B.

If the operator at A wishes to rering the operator at C without disconnecting the cord 13 from theV line, she may operate the ringing key shown in Figure 1. The signals are previously described.

In this description the sending and receiving ringers are shown connected to the four- Yire portion of a four-wire circuit. This signaling system will operate equally as well if the ringers are connected to the two-wire portion of the circuit. This would be done by connecting leads 200 and 201 to leads 202 between the hybrid coil and the jack. The ringers at the other points would be connected in the same manner. This signaling system could also be operated on a two-wire circuit.

Obviously, frequencies other than 1000 or 1400 cycles may be used for signaling purosesvin'the arrangements of the invention and these frequencies have been referred to as desirable for purposes of illustration. Ac-

cordingly, while the invention has been disclosed as embodied in certain specific arrangements which are deemed desirable, it is understood that they are capable of embodiment in many and other widely varied forms without departing fromthe spirit of the invention as defined by the appended claims.

What is claimed'is:V

1. A signaling system comprising a line adapted for transmission in both directions betweentwostations, two` sending ring/ers at' one off said stations for, applying signaling the foregoing thatV currents of different frequencies to said line for transmission in one direction, two receiving ringers at said same station responsive at said different frequenciesrto signaling currents transmitted in the opposite di-I rection overr said line, and means at said station for interconnecting said sending ringers withsaid receiving ringers so that when a sending ringer of one irequency is operative the receiving ringer for that frequency will be disabled.

2. A signaling system comprising a line adapted for transmission k-in both directions between two stations, two sending ringers at the first of said stations for applying signaling currents of different frequencies to said line for transmission from the rst to the second of said stations, two receiving ringers at the first station responsive at said different frequencies to signaling currents transmitted fionrthe second to the first of said stations, two sending ringers at the second of said stations for applying signaling currents of said different frequencies to said line for transmission from said second to said first station, two receiving ringers at said second station responsive at said n it frequencies to signaling currents ransmiitcd from the first to the second of .id stations, and means for interconnecting the sending ringers yat each station with the receiving ringers thereat so that when a sending ringe-r of one of said frequencies is operative the receiving ringer for that frequency at the same station will be disabled.

3. A signaling system comprising a line adapted for transmission in both directions between'two stations, two sending ringers at the first of said stations for appiying signaling currents of di'li'crent frequencies to said line for transmission from the first to the .second of said stations, two receiving ringers at the first station responsive at said different frequencies to signaling currents transmitted from the second to the first of said stations, two sending ringers at the sec-ond of said stations for applying signaling currents of said different frequencies to said line for transmission from said second to iirst station, two receiving ringers at said second station responsive at said di iterent frequencies to signaling crurcnts transmitted from the first to the second of said stations, means for interconnecting the sending ringers at each station with the receiving ringers thereat `so that when a sending ringer of oneoi said frequencies is operative the receiving ringer for that frequency at the same lstation will be disabled, linlr circuits atv each of said stations for extending said line, means controlled bythe association and disassociation of said link circuits with said line for controllinggsaid ringers, and

signaling means at said stations controlled v by said ringers.

ing current of'sa'id first 4. A 'signaling system' comprising a line adapted for transmission in both directions between stations A and B, means operative when said line is in i-dle condition for transmitting signalingcurrent of a first frequency between stations A and B and for transmitting signaling current of a second frequency between stations B and A, means responsive to the establishment of a connection with said line at A for cutting off said first signaling frequency, means responsive to the subsequent establishment of a connection with said line at Bv for` cutting o said second signaling frequency, means responsive tothe breaking of said connection with said line at B for transmittingsignaling current of said first frequency between B and A, means responsive-to the subsequent breaking of said connection with 4said line-at A for transmitting signaling current ofsaid second frequency between A and B, and signal responsive devices at said stations controlled by sa-id signaling currents of different frequencies.

A signaling system comprising a line adapted for transmission in both directions between stations A and B, means operative when said line is in idle condition for transmitting signaling current of a first frequency 'between stations A and B and for transmitsecond frequencyy ting signaling current of a between stations B and A, means responsive to the establishment of a connection with said line-at A for cutting on' said first signaling frequency, means.r responsive to the subsequent establishment .of a connectionwith said line at B for cutting' off said second signaling frequency, means responsive to the breaking of said connection with said line at B for transmitting signaling current of said first frequency between'B and A, means responsive to the subsequent breaking of said connection with said line at A for transmitting signaling current of said second frequency'betweenA and B, means responsive to the subsequent establishment yof a connection 'with said line at A for cutting off said second signaling frequency'between A and B, means responsive yto the subsequent establishment of a connection with said line at B for cutting off said first signaling frequency between B and A,means` responsive to the subsequent breaking of said connection with said line at vA for transmitting signalfrequency between A and B, means responsiveY to thev subsequent breaking of said connection .with said line at B for transmittingvsignaling current of said second frequency between B and A, and` signal responsive devices at said stations controlled by said signaling currents of different frequencies. f

6. A signaling system comprising a line vadaptedfor transmissionA in both directions between stations A and lB, means operative when said line isin idle condition for trans- 'first frequency from B mitting signaling current of a first frequency between stations A and B and for transmitting signaling current of a second frequency between stations B and A, means responsive to the establishment of a connection with said line at A for cutting oft said first signaling frequency between A and B, signaling means at B responsive to said cutting off of said first signaling frequency between A and B, means responsive to the subsequent establishment of a connection with said line at B for cutting off said second signaling frequency between B and A, signaling means at A responsive to said cutting on of said second signaling frequency between B and A, means responsive to the breaking of said connection with said line at B for transmitting signaling current of said first frequency from B to A, signaling meansat A responsive to said transmission of signaling current of said to A, and means responsive to the subsequent breaking of the connection with said line at A for transmitting signaling current of said second frequency between A and B. Y 7. A signaling system comprising a line adapted for transmission in both `directions between stations A and B, means operative when said line is in idle condition for transmitting signaling current of a first frequency between stations A andL B and for transmitting signaling current of a second frequency between stations B and A, means responsive to the establishment of a connection with said line at A for cutting od said first signaling frequency, means responsive to the subsequent establishment of a connection with said line at B for cutting off said second signaling frequency, means responsive to the breaking of said connection with said line at B for transmitting signaling current of said first frequency between B and A, means responsive to the subsequent breaking of said connection with said line at A for transmitting signaling current of said second frequency between A and B, a second transmistion B and signaling current of the other of said frequencies is being transmitted from station A to station'B.

8. A signaling system comprising a line adapted for transmissionk in both directions between stations Ay and B, means operative when said line is in idle condition for transmitting signaling current of a first frequeqgy between stations A and B and for transmity ting signaling current of a second frequency yto the establishment of quency between sion line adapted A between stations A and first frequency Vsponsive to the subsequent breaking of said between stations B and A, means responsive a connection with said line at A for cutting off said first signaling frequency, meansresponsive to the subsequent establisment of a connection with `said line at B for cutting off said second signaling frequency, means responsive to the breaking of said connection with said line at B for transmitting signaling current of said first frequency between B and A, means responsive to the subsequent breaking of said connection with said line at A for transmitting signaling lcurrent of said second fre- A and B, a second transmisfor transmission in both directions between'stations B and C, said second transmission line being similar to said transmission line between stations A and B, anda link circuit for connecting said transmissionlinestogether at station B, said link circuit comprising a signal controlled by circuits over the contacts of four relays, one pair of said relays being controlled by the signaling currents transmitted from station C to station B and the other pair of said relays being controlled by the signaling currents transmitted from station A to station B, the circuits therefor being so arranged that said signall will be responsive when opposite relays of each pair are released or when both relays of a pair are released.

9. A signaling system comprising a line adaptedy for transmission in both directions between stations A and B, means operative .when said line is in idle condition for transmitting signalingcurrent of a first frequency B and for transmitting signaling current of a second frequency between stations B and A, means responsive to .the establishment of a connection with said line at A for cutting olf said first signaling frequency, means responsive to the subsequent establishment of a connection with said line at B for cutting off said second signaling frequency, mea-ns responsive to the breaking of said connection with said line at B' for transmitting signaling current of said between B and A, means reconnection with said line at A for transmitting signaling current of said vsecond frequency between A and B, a second transmission line adapted for transmission in both directions between stations B and C, said second transmission line being similar to said transmissiondine between stations A and B, and a link; circuit for connecting said transmission lines together at station'B, said'link Y circuit comprising a signal controlled by circuits over the contacts offour relays, one pair of'saidfrelays being controlled by the signalingcurrents transmitted from station C to station B and the other pair of said relays being controlled by the signaling currents transmitted from station A to station B, the

Vline being similar -ing a signal controlled `lays being controllo vlrents transmitted from station.

controlled by the circuits therefor being; soV arranged: that said signal willv be responsive when oppositefrelays of each pair are released or whenfboth relays of a pair are released, another link cir-` 10. A signaling system comprising a line j adapted for transmission inkboth directions between stations A and B, means operatine when said line is in idlev condition for transmitting signaling current ofazfir'st frequency between stations A and B and for transmitting signaling current of asecond frequency between stations B and A., means responsive to theestablishment of aconnection withsaid line aty A for cutting; olf said? first signaling frequency, means responsivefto theisubsequent establishment of a connection with said line at B for cutting olf said second:l signaling frequency, means responsive' tov they breaking of said connection with saidv linel at B; for transmitting signaling current of said first frequency between B and A, means-responsive to the subsequent breakingof'said connection with said line atv A vfor transmitting signaling current of said' second frequency between A and B, a second transmissionV line adapted for transmission in both directions between stations B and C, said' second transmission to saidl transmission line between stations A and B, and a link circuit for connecting said transmission linesy together at station B, said' link circuit comprisby' circuits. over the contacts of four rela As, one pair of said reby the signalingjcur- C to station B and the other pair of said relays being signaling currents transmitted. from station A to station B', the circuits therefor being so arranged that said signal will be responsive when opposite relays of each pair are released or when both relays of a pair are released, a second link circuit at station C for extending the connection, said second link circuit beingv similar tosaid link circuit at station B, a switching trunk adapted to be connected to said secondi link circuit at station C, and means operative when said link circuit is connected to said switching station B to station when said line is in idle condition for transmitting signaling current of a first frequency between stations A and B and for transmitting signaling current of a second frequency between stations B and A, means responsive to the establishment of a. connection with said line at A for cutting oii' said first signaling frequency, means responsive to the subsequent establishment of a connection with said line at B for cutting 0E said second signaling frequency, means responsive to the breaking of said connection with said line at B for transmitting signaling current of said iirst frequency between B and A, means responsive to the subsequent breaking of said connection with said line at A for transmitting signaling current of said second frequency between A and B, a second transmission line adapted for transmission in both directions between stations B and C, said second transmission line being similar to said transmission line between stations A and B, and a link circuit for connecting said transmission lines vtogether at station B., said link circuit comprising a signal controlled by circuits over the contacts of four relays, one pair of said relays being controlled by the signaling currents transmitted from station C to station B and the other pair of said relays being controlled by the signaling currents transmitted from station A to station B, the circuits therefor being so arranged that said signal will be responsive when opposite relays of each pair are released or when both relays of a pair are released another link circuit at station A for establishing connection with said line thereat, and a signal in said last mentioned link circuit controlled byy circuits over the contacts of two relays, said relays being controlled by the signaling currents transmitted from station B to station A and from station C to station A, said circuits therefore being so arranged that said signal will respond when either of said relays are released, a link circuit at station C for extending the connection to a subscribers line, said last mentioned link circuit being similar to said link circuit at station B, and means whereby said subscriber may control the application of signaling currents of said different frequencies from station Cto station A.

In testimony whereof, we have signed our names to this specification this 19th day of November, 1931.

LEONARD G. ABRAHAM. ELLIOT M. STAPLES. 

